Regulation of milk letdown involves maternal and offspring contributions. This proposal is concerned with 3 problems: the mechanism by which fasted pups increase their intake, the mechanism by which maternal sensitivity to suckling changes, thereby altering milk release, and the inhibitory effect of stress on milk release. The negative intraoral pressures exerted during sucking by the pups will be recorded via a chronic oral cannula. Changes in the frequency, amplitude and duration of the intraoral pressures will be correlated with alterations in milk intake in fasted rats. Milk ejection will be estimated by pups' weight changes. The aspects of changes in maternal sensitivity to suckling that will be examined are: (i) altered transmission of the suckling signal to the hypothalamus for oxytocin release; (ii) altered sensitivity of the myoepithelial cells to oxytocin. Activities in certain ascending neural paths will be recorded at the level C1 of the spinal cord. The sensitivity of myoepithelial cells to oxytocin will be measured by changes in intramammary pressure. The study of stress on the inhibition of milk release will focus on the effect of the ensuing emotional arousal, that of the adrenocortical hormones and that of the sympathetic-adrenomedullary system. The long term objectives of this study are to focus on the adaptation of the nursing dyad to prolonged exposure to adverse environmental conditions such as hypoxia, hypogravity. In particular, studies will be conducted 1) to delineate the severity of environmental stress to which mother and young can be exposed without ill effect to the offspring, and 2) to determine whether the inhibitory effect of stress on milk letdown can be alleviated in order to maintain normal milk release, thus providing for the proper nutritional need of the litter.